1. Field of the Invention
This invention relates to electrical and/or optical cables and, in particular to wireline cables having optical fiber(s) therein.
2. Description of the Related Art
In the petroleum industry, wireline cables are used to support tools, provide power, and collect data downhole from well-bores. In the case of data collection, the use of optical fibers in electric and/or optical cable offers the potential to carry greater amounts of information than conventional conductors. This is important since at a set diameter, factors such as maximizing data transfer, cable strength, power capacity, and environmental durability are critical to optimum cable design. Optical fibers present certain difficulties such as degradation due to hydrogen exposure, particularly at high temperatures, lack of comparable stretch/strain characteristics as compared with other cable elements, the possibility of volatilization of volatile organic compounds (VOCs) in coatings or other polymeric protective layers on the optical fibers, and hydrolytic attack against glass in the presence of water.
Electrical and/or optical cables, such as those used in oilfield wireline operations, often include members that provide tensile strength to the cables. Historically, one or more layers of wire comprising a plough or ferritic steel are applied to the outer surfaces of such cables to form strength members. Metallic strength members in cables stretch under load and then return to their original length. Polymeric (un-crosslinked) materials in wireline cables stretch but do not return to their original lengths. Existing designs for fiber optic conductors used in wireline cables have incorporated several measures to protect the fiber optic elements. For example, Schlumberger's patent “Fiber Optic Cable and Core” (U.S. Pat. No. 4,375,313) places helically wound optical fibers around a polymeric core with additional polymeric material applied over the optical fibers. In this type of design, the polymeric material stretches along with the strength members, and the optical fibers' helical configuration allows them to extend with that stretch. However, when the elongation stress is removed from the cable the polymeric material does not return to its original length, which leads to local stress points and causes signal attenuation. Optical fibers have markedly different deformation characteristics than a cable's metallic strength members and limited ability to stretch. Thus, a typical mechanical limitation for acceptable performance of optical fiber based cables is the amount of stretch a cable can withstand. The present invention provides cables comprising optical fiber(s) in conjunction with metallic conductors in configurations that avoid mechanical and durability limitations present in the prior art.